Implement poll support. Polled files are indexed using fh in a RBtree rooted at fuse_conn->polled_files. All pollable files shouldhave unique fh as that's how notifications are matched to files. Ifduplicate fhs are detected, FUSE spits out warning message. Poll willmalfunction but otherwise it will work fine.

Client should send FUSE_NOTIFY_POLL notification once after processingFUSE_POLL which has FUSE_POLL_SCHEDULE_NOTIFY set. Sendingnotification unconditionally after the latest poll or everytime filecontent might have changed is inefficient but won't cause malfunction.

fuse_file_poll() can sleep and requires patches from the followingthread which allows f_op->poll() to sleep.